1. Field of the Invention
This invention relates to tubular furnaces for indirectly heating hydrocarbons. More particularly, the invention concerns improvements in both the furnace apparatus and in methods of furnace operation which can minimize problems caused by coking in the tubes and can facilitate decoking procedures.
2. Discussion of the Art
Pyrolysis of hydrocarbons, such as cracking of naphtha, gas oils, or C.sub.2 -C.sub.4 alkanes to produce ethylene and propylene, is commonly accomplished in vertical tube furnaces. An example is found in U.S. Pat. No. 4,479,869 to Petterson et al., which concerns a pyrolysis process capable of using a range of hydrocarbon feedstocks.
The vertical tube furnace described in U.S. Pat. No. 4,412,975 to Parizot et al. and its divisional U.S. Pat. No. 4,324,649 is similar to those being commercially used to crack hydrocarbon feedstocks to ethylene. However, this design has the disadvantage that the furnace tubes cannot be inspected or cleaned without complete shutdown and isolation of the furnace to allow vessel entry.
Cracking of light hydrocarbon feeds such as ethane, propane, butane, or mixtures thereof requires hotter furnace temperatures, for example at over 1150.degree. C. [over 2000.degree. F.] in the upper furnace region as described in Petterson '869, compared to temperatures for naphtha feedstock. Coke laydown in the furnace tubes becomes a more serious problem at severe conditions, impeding the process flow. Coke particles can also fall within the tube, obstructing the tube itself or the feed inlet. If the tube or inlet becomes completely blocked, normal decoking operations are ineffective and the furnace must be shut down.